Copending application Nos. 024,769 and 024,770, both filed Mar. 11, 1987, (and issued on Aug. 22, 1989 as Pat. Nos. 4,859,859 and 4,859,858, respectively) disclose novel gas analyzers of the non-dispersive type which operate on the premise that the concentration of a designated gas can be measured: (1) by passing a particularized beam of infrared radiation through the gas, (2) and ascertaining the attenuation of the energy in a narrow band absorbable by the designated gas with a detector capable of generating an attenuation proportional electrical output signal.
Medical applications of these gas analyzers are among the more important and perhaps the most significant of these is the monitoring of tidal carbon dioxide, i.e., the concentration of carbon dioxide in a patient's exhalations.
This expired carbon dioxide level can be employed by medical personnel to control the operation of a mechanical ventilator hooked up to the patient to assist him in breathing. In certain major surgical procedures, the ventilator completely takes over the breathing function for the patient.
In a typical medical application of the gas analyzers just described, an airway adapter is employed to connect a tube inserted into the patient's trachea to the plumbing of a mechanical ventilator (not shown). The airway adapter also confines the expired gases to a flow path with a precise, transverse dimension; and it furnishes an optical path between an infrared radiation emitter and an infrared radiation detector unit, both components of an infrared transducer.
The infrared radiation traverses the gases in the airway adapter where it is attenuated because part of the radiation is absorbed by the designated gas being analyzed. The attenuated beam of infrared radiation is then filtered to eliminate energy of frequencies lying outside a narrow band which is absorbed by the gas being measured. The infrared radiation in that band impinges upon a detector which consequentially generates an electrical signal proportional in magnitude to the intensity of the infrared radiation impinging upon it.
This novel arrangement allows the analysis to be performed at a location immediately adjacent the patient instead of the samples being transmitted to a more remote location for analysis as is commonly done in other gas analyzers. This is an advantage because distortion attributable to the transmission of the sample to the remote location is eliminated. Also eliminated are problems commonly encountered with the lines through which the sample is routed--poor dynamic response, water in the line, etc.
Nevertheless, the system just described has the disadvantage that it is useful only in those applications in which the patient is being intubated; i.e., assisted in breathing or breathed for by a mechanical ventilator. There still exists, as a consequence, a need for gas analyzers with the advantages and attributes of those described in the preceding paragraph but of greater utility in circumstances where the patient is breathing satisfactorily on his own and therefore does not require intubation but is in a situation where it is necessary or advantageous to monitor his breathing. One example is where a local anesthetic is being administered. In these and comparable situations, it is highly desirable to avoid the trauma involved in installing a tracheal tube as would be required if the previously disclosed gas analyzer apparatus were employed.